Methods for etching of substrates are widely used in the fabrication of semiconductors and electronic components. Generally, etching processes are used in the fabrication of building metal layers and structures on the substrates. There are two general approaches to create patterned metal on a substrate. The “subtractive” or “etch-back” approach is to first deposit metal over the entire substrate surface followed by subsequent patterning of a resist etch mask on top of the metal and then selectively removing the metal in unwanted areas by wet or dry etching. The “additive” or “liftoff” approach is to do metal liftoff in which resist is first patterned on the substrate surface followed by subsequent deposition of metal by sputter deposition or evaporation method. The sacrificial resist layer is then dissolved in a suitable solvent, lifting off the metal on top of the resist and leaving only metal in the resist opening areas on the substrate. The “liftoff” technique allows easy creation of metal patterns comprising different metal layers whereas selective removal of all the metal layers in the “etch-back” approach is not always easy and straightforward.
To facilitate metal deposition and subsequent clean metal liftoff, an overhang resist edge profile is often needed to prevent metal from being deposited on sidewalls of the resist and connected to the metal deposited on the substrate in patterned areas, and to allow for the solvent to reach and dissolve the sacrificial resist layer. This can be accomplished by several conventional techniques.
The simplest technique is to coat and then expose a single photoresist layer on a substrate through an imaging mask. A positive photoresist is one in which the exposed region will get dissolved in a developer whereas a negative photoresist works the opposite way. Owing to the gradual decrease of light intensity through absorption in photoresist during image exposure, the portion of resist closest to the surface sees the highest intensity of light and the bottom part has least. Upon resist development, positive photoresist naturally gives a positive slope of resist profile along edge of the opening and negative photoresist gives a negative slope instead. The negative slope in the negative resist edge profile provides the necessary resist overhang for clean metal liftoff. However, a drawback of negative photoresists is the swelling of resist around the development pattern, which makes the dimension control difficult.
In one prior art technique represented schematically in FIG 1a, a single layer of positive photoresist 110 is used on top of an underlying substrate material 100 and as generally shown in J. M. Shaw et al., “Negative photoresists for optical lithography”, IBM Journal of Research and Development, vol. 41, nos. 1/2, 1997, FIG. 3a. Upon an image exposure and resist development in a suitable developer, a resist opening with a positive slope of resist edge profile is created. The resist opening exposes portion 102 of the underlying substrate. An isotropic dry or wet etch is then used to remove the exposed portion 102 of the underlying substrate material 100 in the resist opening to an etched surface 112 of a desired etch depth using the patterned positive photoresist 110 as the etch mask. This is illustrated in FIG 1b. The isotropic etching creates etch undercuts 114 and 116 around the photoresist opening. For this approach of using a single layer of positive photoresist 110 and relying on the etch undercuts 114 and 116 to do a subsequent metal liftoff to work, the total thickness of the deposited metal 120 shown in FIG. 1c has to be smaller than the etch depth or otherwise the metal 120 deposited on the etched substrate surface 112 in the photoresist opening would be connected to the same metal 120 deposited on the sidewalls of the photoresist opening and therefore lead to shorted metals and difficulties in subsequent metal liftoff. This limits the total thickness of the metal that can be deposited on the etched surface. The metal liftoff is achieved by dissolving the photoresist layer 110 in a suitable solvent such as acetone, thereby lifting off the deposited metal 120 on top of the patterned photoresist 110 and leaving only the deposited metal 120 on top of the etched surface 112 of the underlying material 100. The finished structure is shown in FIG 1d. 
In another prior art technique shown in FIG. 2a, the process makes use of a single layer of negative photoresist 210 on top of an underlying substrate material 200. Upon an image exposure and resist development in a suitable developer, a resist opening with a natural negative slope of resist edge profile is created. The resist opening exposes portion 202 of the underlying substrate. Owing to the negative slope of the resist edge profile, resist undercuts 204 and 206 are naturally present and facilitate subsequent metal liftoff. An isotropic or anisotropic dry or wet etch is then used to remove the exposed portion 202 of the underlying substrate material 200 in the resist opening to an etched surface 212 of a desired etch depth using the patterned negative photoresist 210 as the etch mask. This is illustrated in FIG. 2b. The etching may create additional etch undercuts 214 and 216 around the photoresist opening. As shown in FIG. 2c, the total deposited metal thickness is to be no greater than the combined thickness of the negative photoresist 210 and the etch depth of the underlying material 200 for clean metal liftoff. Unlike the prior art shown in FIGS. 1a to 1d, this method allows the deposited metal 220 to be thicker than the etch depth of the underlying substrate material 200 and still gives clean metal liftoff. The metal liftoff is accomplished by dissolving the photoresist layer 210 in a suitable solvent such as acetone, thereby lifting off the deposited metal 220 on top of the patterned photoresist 210 and leaving only the deposited metal 220 on top of the etched surface 212 of the underlying material 200. The laterally etched regions 222 and 224 in the finished structure shown in FIG. 2d are generally wider than the corresponding regions 122 and 124 in FIG. 1d in the prior art that used a single layer of positive photoresist 110 for the same etch depth and the same photoresist adhesion to the same underlying material 100 and 200. It is a disadvantage of this approach that makes use of a resist edge profile with such a negative slope around the photoresist opening in etching prior to metal deposition if one of the requirements is to minimize the laterally etched regions 222 and 224 around the deposited metal 220 on the etched substrate surface 212.
U.S. Pat. No. 4,212,935, describes another prior art technique wherein a single positive photoresist layer is also used but a chemical (e.g. chlorobenzene, fluorobenzene, bromobenzene, xylene or toluene) treatment is done either before or after the pattern exposure of the photoresist layer and either before or after a photoresist bake. This retards the developer attack in the surface region of the photoresist layer affected by the chemical modification. The resultant photoresist 310 has an undercut or overhang edge profile around the photoresist pattern opening as illustrated in FIG. 3a. The resist undercuts 304 and 306 under the photoresist overhangs 307 and 308 facilitate subsequent metal liftoff. An isotropic or anisotropic dry or wet etch is then used to remove the exposed portion 302 of the underlying substrate material 300 in the resist opening to an etched surface 312 of a desired etch depth using the patterned positive photoresist 310 as the etch mask. This is illustrated in FIG. 3b. The etching may create additional etch undercuts 314 and 316 around the photoresist opening. As shown in FIG. 3e, the total deposited metal thickness is to be no greater than the combined thickness of the photoresist under the resist overhangs and the etch depth of the underlying material 300 for clean metal liftoff. Also unlike the prior art shown in FIGS. 1a to 1d, this method allows the deposited metal 320 to be thicker than the etch depth of the underlying substrate material 300 and still gives clean metal liftoff. The metal liftoff is accomplished by dissolving the photoresist layer 310 in a suitable solvent such as acetone, thereby lifting off the deposited metal 320 on top of the patterned photoresist 310 and leaving only the deposited metal 320 on top of the etched surface 312 of the underlying material 300. The laterally etched regions 322 and 324 in the finished structure shown in FIG. 3d are also generally wider than the corresponding regions 122 and 124 in FIG. 1d in the prior art that used a single layer of positive photoresist 110 for the same etch depth and the same photoresist adhesion to the same underlying material 100 and 300. It is a disadvantage of this approach that makes use of a photoresist edge profile with such an overhang around the photoresist opening in etching prior to metal deposition if one of the requirements is to minimize the laterally etched regions 322 and 324 around the deposited metal 320 on the etched substrate surface 312.
Further developments in the prior art have focused on an image reversal photoresist process to produce a re-entrant resist edge profile needed for clean metal liftoff. There are two ways to create the image reversal effect shown in the prior art. In a first method such as that generally described in U.S. Pat. No. 4,775,609 and in IBM Journal of Research and Development, vol. 41, nos. 1/2, 1997, FIG. 4b, and represented schematically herein in FIG. 4a, a single layer of positive photoresist 410 is first selectively exposed with near UV or violet irradiation through an imaging mask. It is then treated with a gaseous base such as ammonia. The wafer is subsequently exposed to vacuum to remove the excess base. It is then followed by a blanket exposure without a mask to near UV or violet light. A high temperature bake (known as a “reversal” bake) is next performed before the photoresist is finally developed in a suitable aqueous alkali developer. Those areas that received the first near UV or violet irradiation through the imaging mask remain and those areas that were blanket exposed but did not receive the first near UV or violet irradiation are removed by the developer, thereby resulting in the inverse of the original mask pattern. Alternatively, a single layer of an image reversal photoresist such as the AZ 5200E series resists manufactured by AZ Electronic Materials USA Corporation can also be used to achieve the same image reversal effect without the need for a gaseous ammonia treatment and vacuum. The image reversal photoresist film 410 is first selectively exposed with near UV or violet irradiation through an imaging mask and then goes through a “reversal” bake, which makes the exposed image areas less soluble in basic developers than the unexposed image regions. The resist film is then blanket exposed without a mask with near UV or violet irradiation to expose both the previously exposed and unexposed areas. Upon the removal of the unexposed image areas in a suitable developer, a negative image of the original mask with a re-entrant resist edge profile is formed as shown in FIG. 4a. In either method, the slope of the photoresist sidewalls can be tailored from positive to negative by carefully controlling the reversal bake temperature, the bake time and the resist development time. The re-entrant resist edge profile having undercuts 404 and 406 forms the resist overhang structure needed for subsequent metal liftoff. In this approach, an isotropic or anisotropic dry or wet etch can be used to subsequently remove the exposed portion 402 of the underlying material 400 in the resist opening to an etched surface 412 of a desired etch depth using the patterned positive resist 410 as the etch mask. This is illustrated in FIG. 4b. The etching may create additional etch undercuts 414 and 416 around the photoresist opening. As shown in FIG. 4c, the total deposited metal thickness is to be no greater than the combined thickness of the resist under the resist overhangs and the etch depth of the underlying material 400 for clean metal liftoff. Also unlike the prior art shown in FIGS. 1a to 1d, this method allows the deposited metal 420 to be thicker than the etch depth of the underlying substrate material 400 and still gives clean metal liftoff. The metal liftoff is accomplished by dissolving the photoresist layer 410 in a suitable solvent such as acetone, thereby lifting off the deposited metal 420 on top of the patterned photoresist 410 and leaving only the deposited metal 420 on top of the etched surface 412 of the underlying material 400. The laterally etched regions 422 and 424 in the finished structure shown in FIG. 4d are also generally wider than the corresponding regions 122 and 124 in FIG. 1d in the prior art that used a single layer of positive photoresist 110 for the same etch depth and the same photoresist adhesion to the same underlying material 100 and 400. It is a disadvantage of this approach that makes use of such a re-entrant resist edge profile around the photoresist opening in etching prior to metal deposition if one of the requirements is to minimize the laterally etched regions 422 and 424 around the deposited metal 420 on the etched substrate surface 412. Another known problem in this approach is the difficulty in controlling of the amount of photoresist undercuts 404 and 406 in the re-entrant resist edge profile.
When thick photoresist is needed to provide good resist step coverage on a highly non-planar surface and yet be able to simultaneously give good pattern resolution, or when independent control of resist overhangs or undercuts is needed, a multi-layer resist system can be used. Independent control of the size of the opening in the top resist and the undercuts in the bottom resist can be accomplished with multi-layer resist methods. Many combinations or permutations using two or three layers of different materials have been reported. Some of them are described below.
Typically a bilayer resist process comprises a bottom resist layer that contacts the surface of the substrate and a top resist layer overlying the bottom resist layer. The top resist layer is first lithographically defined. The pattern in the top resist layer is then transferred to the bottom resist layer by etching the bottom resist in plasma such as oxygen reactive ion etching (RIE). These bilayer resist schemes are generally used when both good step coverage and good resolution are needed. The bottom resist layer is typically made relatively thick to first planarize the substrate surface and the top resist layer is made relatively thin to provide the needed resolution for fine features to be patterned in the top resist layer by lithographic means. For these techniques to work, it is important to have the top resist layer being essentially unaffected by plasma etching relative to the bottom resist layer. One example of how to make the top resist layer more resistance to plasma etching relative to the underlying resist layer is given in U.S. Pat. No. 5,318,877. These methods, however, generally do not result in sufficient resist overhangs needed for clean metal liftoff for a thick metal layer and the resultant bilayer resist edge profile is similar to that in the prior art that utilizes a single layer of positive photoresist described earlier and illustrated in FIGS. 1a to 1d. In these cases the total deposited metal thickness is limited to no greater than the etch depth of the underlying substrate material.
To create an overhang resist edge structure, one prior art technique involves a bilayer photoresist process comprising two layers of positive-working photoresist as described in detail in U.S. Pat. No. 5,360,698. Care must be taken to prevent intermixing of the two similar photoresist materials. Typically, after the coating of the bottom photoresist layer, plasma etch or thermal treatment is used to alter the surface characteristics of the bottom resist layer to produce a buffer layer which prevents the intermixing. This process allows the top resist to be coated uniformly and maintains the distinction between the two layers. Two resist materials may be chosen such that they either exhibit different dissolution rates in the same developer or else they use mutually exclusive developers. In this case, an overhang resist structure can be produced. Depending upon the treatment conditions used to form the buffer layer, it may be necessary to use a two step development process with an intermediate etch step to remove the buffer layer.
In another prior art technique as described in U.S. Pat. No. 5,360,698, the overhang resist edge structure uses two separate resist layers, the bottom being deep UV patternable and the top being patternable at an appropriate wavelength other than deep UV and having low optical transmission properties at the wavelength used in a deep UV exposure step and characterized by decreased solubility and/or increased crosslink density after such deep UV exposure. The development of the bottom resist removes some of the bottom resist layer that is under the top resist resulting in an overhang resist edge structure for metal liftoff.
In yet another prior art technique, generally described in U.S. Pat. No. 6,495,311 and schematically represented herein in FIG. 5a, a bilayer resist process consisting of a first, bottom resist layer 510 such as polymethyl-glutarimide (PMGI) (U.S. Pat. No. 6,495,311 B1) or polymethyl-methacrylate (PMMA) (H. Kluak et al., “Fast Organic Thin-Film Transistor Circuits, IEEE Electron Device Letters, vol. 20, no. 6, June 1999, pp. 289) and a second, top photoresist layer 520 is used. Using standard photolithographic techniques, the top layer photoresist is exposed through an imaging mask and developed with a suitable developer. The developer may remove portion of the lower resist layer through the opening in the top photoresist layer. Alternatively, the lower resist layer can be developed separately with a second developer that dissolves only the lower resist layer but not the upper photoresist. An oxygen plasma descum or reactive ion etching step may be necessary to remove the intermixed interfacial layer between the two layers of resist before the bottom resist layer is developed. Using either developing step, the bottom resist layer is developed to expose portion 502 of the underlying substrate 500 through the opening in the top photoresist layer. Undercuts 504 and 506 in the bottom resist layer are formed during the development of the bottom resist layer. Subsequent steps illustrated in FIGS. 5b to 5d for utilizing the resultant bilayer overhang resist structure for etching the exposed portion 502 of the underlying material 500 and subsequently lifting off the deposited metal 530 on top of the top photoresist layer 520 are similar to those in other prior art described previously in FIGS. 2b to 2d, 3b to 3d and 4b to 4d. As shown in FIG. 5c, the total deposited metal thickness is to be no greater than the combined thickness of the bottom resist layer 510 and the etch depth of the underlying material 500 for clean metal liftoff. Upon metal liftoff in a suitable solvent such as acetone, only the deposited metal 530 is left on top of the etched surface 512 of the underlying material 500. The laterally etched regions 522 and 524 in the finished structure shown in FIG. 5d are also generally wider than the corresponding regions 122 and 124 in FIG. 1d in the prior art that used a single layer of positive photoresist 110 for the same etch depth and the same photoresist adhesion to the same underlying material 100 and 500. It is a disadvantage of this approach that makes use of a bilayer resist process with such an overhang resist edge profile in etching prior to metal deposition if one of the requirements is to minimize the laterally etched regions 522 and 524 around the deposited metal 530 on the etched substrate surface 512.
Bilayer resists are less cumbersome than trilayer resists. But if they do not work out for a specific application, trilayer methods may be considered. A typical one of the multi-layer methods is a three-layer method which provides upper and lower resist layers and an intermediate layer disposed between them. A pattern is transferred from the upper layer to the intermediate layer and from the intermediate layer to the lower layer with the use of a RIE process. The intermediate layer prevents the upper and lower resist layers from interacting with each other and provides etch resistance when the lower layer is subjected to the RIE process. To achieve this function, the intermediate layer is usually made of spin on glass (SOG, i.e. organic silicon glass) or metal as in U.S. Pat. No. 5,665,251. This method allows for the formation of an overhang resist edge profile needed for clean metal liftoff.
There are other methods of making bilayer liftoff masks involving, for example, electron beam lithography such as described in U.S. Pat. No. 6,218,056 B1. However, such techniques require the use of a separate, expensive electron beam lithography tool and lose the advantage of doing all mask patterning steps all optically with a conventional photolithography tool such as a stepper or a mask aligner.
The conventional techniques and prior art described above provide good ways to form the necessary overhang or re-entrant resist edge profile in an opening for metal liftoff. However, such methods have limitations. With the exception of the prior art that uses a single layer of positive photoresist, as illustrated in FIGS. 1a to 1d, and a bilayer resist process wherein the bottom resist is patterned by anisotropic etching in oxygen RIE using the top resist layer as the etch mask and in either case no resist overhang is present, all other aforementioned prior art involve the first creation of a resist overhang structure before a subsequent etching of the underlying material is made. Without a resist overhang for clean metal liftoff, the total thickness of the deposited metal is limited to no greater than the etch depth of the underlying substrate. When resist overhangs or undercuts are present, however, the subsequent etch undercuts of the underlying substrate around the deposited metal pattern on the etched substrate surface are naturally large compared to ones with straight resist sidewalls. Accordingly, further improvements are needed. Particularly, improved methods are needed that address the limitations of the prior art techniques while providing sufficient step coverage on a highly non-planar substrate surface and allowing for the total deposited metal thickness to be greater than the etch depth of the underlying substrate. Improvements are also needed that provide for scaling the etch undercut regions.